All tRNAs have two functions: to chemically link to a specific amino acid and to recognize a codon in mRNA so that the linked amino acid can be added to a growing peptide chain during protein synthesis. In general there is at least one aminoacyl-tRNA synthetase for each of the twenty amino acids. A specific aminoacyl-tRNA synthetase links an amino acid to the 2' or 3' hydroxyl of the adenosine residue at the 3'-terminus of a tRNA molecule. Once its correct amino acid is attached, a tRNA then recognizes a codon in mRNA, thus delivering its amino acid to the growing polypeptide chain. These enzymatic functions are critical to gene expression (Neidhart et al. (1975) Annu. Rev. Microbiol. 29:215-250). Mutations in tRNA synthetases often result in alterations in protein synthesis and in some cases cell death.
Plants like other cellular organisms have aminoacyl-tRNA synthetases. However a complete description of the plant `complement` of aminoacyl-tRNA synthetases has not been published. It is anticipated that plants will likely have at least forty aminoacyl-tRNA synthetases. Plants have three sites of protein synthesis: the cytoplasm, the mitochondrial and the chloroplast. Accordingly, there could be as many as sixty aminoacyl-tRNA synthetases. Based on knowledge of other eukaryotes the cytoplasmic and mitochondrial aminoacyl-tRNA synthetases are expected to be encoded by the same gene. This gene should be nuclearly encoded and produce two alternate products, one with a mitochondrial specific transit peptide, and the other without the mitochondrial targeting signal. The chloroplast is the other site of protein synthesis in plants. Based on a few examples of known plant chloroplast specific aminoacyl-tRNA synthetase genes it appears that these genes are also nuclear encoded. Chloroplast aminoacyl-RNA synthetases would directed to the chloroplast by a transit peptide.
Because of the central role aminoacyl-tRNA synthetases play in protein synthesis any agent that inhibits or disrupts aminoacyl-tRNA synthetase activity is likely to be toxic. Indeed a number of aminoacyl-tRNA synthetase inhibitors (antibiotics and herbicides) are known (Zon et al. (1988) Phytochemistry 27(3):711-714 and Heacock et al. (1996) Bioorganic Chemistry 24(3):273-289). Thus it may be possible to develop new herbicides that target aminoacyl-tRNA synthetases and engineer aminoacyl-tRNA synthetases that are resistant to such herbicides. Accordingly, the availability of nucleic acid sequences encoding all or a portion of these enzymes would facilitate studies to better understand protein synthesis in plants, provide genetic tools for the manipulation of gene expression, and provide a possible target for herbicides.